Seal strip for a suction roll and a method for manufacturing it

ABSTRACT

The invention relates to a suction roll seal strip and a method for manufacturing it from a mixture, which mostly consists of nitrile rubber and graphite. A seal blank is formed from the mixture and hardened at a chosen temperature. The mixture to be hardened includes wax.

FIELD OF THE INVENTION

[0001] The present invention relates to an improved seal strip for asuction roll, which is manufactured by hardening an extruded mass into ashape, most of the mass being polymer and graphite. The invention alsorelates to a method for manufacturing the seal strip.

BACKGROUND OF THE INVENTION

[0002] Suction rolls are used particularly in paper machines, to removewater from the paper web. The suction roll has a perforated shell and astationary suction box inside, so that the part of the jacket next to itis subjected to suction. The seal, i.e. the seal strip is arranged in aholder in the structure of the suction box, this generally being aU-shaped holder, in the bottom of which, and possibly on the sides ofwhich there are loading hoses, for pressing the seal initially againstthe shell. Later, the suction holds the seal against the shell.

[0003] The suction box, which is elongated in the longitudinal directionof the roll, is bounded by long side seals, the first of which, i.e. theseal strip on the wet side, is narrower than the second, i.e. the sealstrip on the dry side. In addition, there are ends seals at the ends ofthe suction box, set at a small gap from the shell. U.S. Pat. No.5,746,891 discloses the seal construction of one such suction roll.

[0004] Seal wear has become a quite important problem. In addition, thenoise and loss of power caused by the seals are important factors.Though wear resistance is thus a primary requirement in seals, a lownoise output and power demand would also be desirable properties.

[0005] Seal strips, seals in brief, are of vulcanized rubber-graphitecompounds. The typical configuration of a seal strip is a continuouspiece, which is the same length as the suction box (3-12 m). Multi-pieceseal strips exist, but these require special machining and installation.The material wears during operation and must be replaced at relativelyfrequent intervals. The nature of the material is that it is a rigid,brittle structure. The seals are sent to the mill in rigid containerswith space for the entire length of the strip. The length and rigidityof the package result in considerable shipping and storage costs. Thebrittleness of the seal material results in a lowered yield inmanufacturing and breakages during shipping and installation.

[0006] During operation, the seal strips are loaded against the innershell of the roll by means of so-called loading hoses. Though theflexibility of the loading means permits some alignment of the sealstrip from end to end, the rigidity of the seal strip prevents localconformity with the roll shell. Therefore all irregularities due to themanufacture of the shell, or deflection can be compensated for only withthe aid of wear. As the seal is intended to be a critical element, itsuseful life is shortened.

[0007] The most common material for constructing a seal strip is avulcanized rubber-graphite compound. It is used in the vast majority ofrolls today.

[0008] Publication U.S. Pat. No. 5,649,719 (Beloit) discloses the use ofgraphite impregnated with resin for the manufacture of a suction rollseal strip. Publication U.S. Pat. No. 4,714,523; Sawyer, discloses aseal-strip construction, in which a PTFE strip is set inside a graphitestrip, in order to improve its sliding properties. Publication U.S. Pat.No. 4,915,787 (Cline Company) discloses a seal-strip construction, inwhich a long seal strip is formed from short pieces, the ends of whichare specially shaped to form overlapping joints between the strips.Publication U.S. Pat. No. 5,876,566 (Appleton) discloses a seal strip,which is formed from nitrile rubber, graphite, carbon black, PTFE, andoptionally phenol resin. U.S. Pat. No. 2,893,487 (Beloit) discloses asuction-box solution, in which, in the rearward seal strip, grooving isformed against the shell on the trailing edge of the strip. This reducesthe detrimental effects, such as noise, of a sudden change in pressure.

[0009] A combination of graphite and rubber is desirable because itprovides good lubricating properties, does not wear excessively innormal operation, and is easy to manufacture. It can be manufactured inall the lengths required by papermaking equipment.

[0010] All rubber graphite seals strips are made in the followingmanner:

[0011] A rubber graphite mixture is milled to an acceptable consistencyfor extrusion

[0012] The compound is extruded to the approximate shape of the endproduct, or of a part product

[0013] The shaped strips are vulcanized to the final rigid condition

[0014] The rigid strips are machined to their final shape.

[0015] Fundamentally, all rubber graphite strips have the same physicalcharacteristics, are made the same way, and perform similarly. They areall limited by shipping and handling constraints, due to their rigid andbrittle nature.

[0016] The usual vulcanizing temperature is about 150° C. and theprocessing time is 3 hours. Additives are used, for instance, to createa suitable chemical structure in the end product. Certain additives, forexample, sulphur, affect the number of carbon links. Some additives actas accelerating agents.

[0017] Other materials have been tried as seal strips, but none havebeen as successful are rubber graphite.

SUMMARY OF THE INVENTION

[0018] The present invention is intended to create an improvement overthe known seals.

[0019] Accordingly, a suction roll seal strip, which is manufacturedfrom a mixture, which mostly consists of nitrile rubber and graphite, ischaracterized in that the mixture includes wax. The amount of wax in themixture is set in such a way that the flexibility of the seal strippermits it to be bent onto a reel, the radius of which is less than 130cm.

[0020] A method for manufacturing a suction roll seal strip from amixture, which mostly consists of nitrile rubber and graphite, and fromwhich mixture a seal blank is formed, which is hardened at a chosentemperature, is characterized in that the mixture to be hardenedincludes wax. In a specific embodiment the amount of wax is 1-15%,preferably 2-4%. The amounts of nitrile rubber and graphite are withinthe ranges: nitrile rubber 30-60% volume, and graphite 30-60% volume.

[0021] The method is characterized in that the hardening temperature isin the range 140-160° C., preferably in the range 145-155°, and thehardening time is in the range 3-5 hours. The mass includes 1-15%sulphur or peroxide and the graphite used is natural graphite.Alternatively, the graphite used is synthetic graphite.

[0022] The method can be further characterized in that the melting pointof the wax used is more than 100° C., preferably within the range110-150° C.

[0023] The amount of wax is arranged to create flexibility in the sealstrip, which will permit it to be rolled onto a reel with a radius ofless than 1.5 m.

[0024] The seal strip according to the invention is so flexible that itcan be rolled up. Besides flexibility, the invention gives a sealquieter operation and reduces the power it requires. Shipping andhandling damage is reduced, as the material is less brittle thanpreviously.

[0025] These and other features and advantages of the invention will bemore fully understood from the following detailed description of theinvention taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the drawings:

[0027]FIG. 1 shows a schematic view of the construction of a suctionbox;

[0028]FIG. 2 shows a comparison of the wear in a conventional seal stripand in a seal strip (FlexSeal) according to the invention;

[0029]FIG. 3a shows a comparison of the power required by a conventionalseal strip and by a seal strip (FlexSeal) according to the invention;and

[0030]FIG. 3b shows a comparison of the noise levels when using aconventional seal strip and when using a seal strip (FlexSeal) accordingto the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] With reference to FIG. 1, the reference number 10 indicates ageneral view of a suction-box arrangement in a paper machine. Thisincludes a stationarily supported suction box 12 located inside aperforated cylindrical roll 14, which rotates around the suction box 12.The cylindrical roll 14 has an inner surface 16 and an outer surface 17and it is manufactured from, bronze, stainless steel, or a similarmaterial. The suction box 12 has a gap opening 18 against thecylindrical roll 14, with U-shaped gaps 21 and 23, in practiceparticularly holders, in which seal strips 20 and 22 are placed, at bothof its edges. The latter of these is generally the wider. The sealstrips 20, 22 are loaded from below by corresponding loading hoses 24and 25. The seal strips 20 and 22 wipe against the inner surface of thecylindrical roll 14 as it rotates.

[0032] The inner part of the suction box 12 is connected to a vacuumsource (not shown), so that a suction effect acts on the inner surface16 of the cylindrical roll 14 in the area of the gap 18. The seal strips20 and 22 extend over the length of the suction box 12 (thus coveringthe width of the web being dried) while there are special end seals (notshown) at the ends of the suction box 12.

[0033] Previously, the following recipe, for example, was used in themanufacture of the mass:

EXAMPLE 1

[0034] % weight a) Polymer Nipol N612B 20 b) Nipol 1312 Liquid Rubber 10a and b have together 30% acrylonitrile rubber stearic acid 0.6magnesium oxide 1.5 graphite 56 accelerator MBTS 0.4 sulphur 12

[0035] The mass was mixed, ground carefully, and extruded through amould, to create a strip blank. The blank was transferred to anautoclave, in which the processing lasted for 3 hours at 150° C. Duringprocessing, the chemical reactions were completed. The result obtainedfrom the mould was a somewhat flexible solid piece (density 1.63 g/cm³),which was transported fully extended, because the strip, with across-section of about 1.9×4.8 cm, could not be bent to a practicablecurve.

[0036] In the invention, an solution to the shipping and storageproblems of seal strips has been sought, as these are mainly due to therigidity of the known strips. These problems are solved by using a newseal material, in which the following substances are used (as %volumes): relative specific gravity NBR rubber 30-60%, preferably 40-50%0.98 Graphite 30-60%, preferably 40-50% 2.25 Wax 1-20%, preferably 2-6%0.97

[0037] It is advantageous to use small amounts of known auxiliarysubstances and additives. These include vulcanizing agents, such assulphur; accelerating agents, such as peroxides; stabilizing agents,such as stearic acid; machining agents, such as MgO and ZnO.

[0038] In one manufacturing series according to the invention, thefollowing recipe was used:

EXAMPLE 2

[0039] % weight PHR % volume NBR rubber 28 100 45 graphite 66 235 46 wax3 10.5 4.5 vulcanizing agent 3 10 4.5

[0040] The product was extruded, vulcanized, and machined in a knownmanner. The vulcanizing temperature was 150° C., the processing timebeing 3 hours. At a lower temperature the reactions are slower and at ahigher temperature faster, which in turn affects the processing time.

[0041] In the composition of the basic material, there is NBR rubber(acrylonitrile rubber), which is a copolymer of butadiene andacrylonitrile. The NBR rubber used in the example was Nipol N612B, ZeonChemicals. The graphite used in the example was synthetic, but it canalso be natural graphite.

[0042] The following table gives the most usual sizes of seal strips andtheir bending radii minimum size (cm) bending radius (cm) Example 1 1.9× 4.76  infinite (prior art)  7 × 4.76 infinite Example 2 1.9 × 4.76 30-100 (invention) 2 × 4.1 30-100  7 × 4.76 <130 7 × 4.4 <130

[0043] The essential new component is wax, which is preferably N,N′-ethylene bis-stearamide wax, for example, Advawax, Rohm & Haas, orAcrawax C, Lonza, or some other corresponding EBS wax (ethylenebis-stearamide wax), or some other bis-stearamide wax, or hydroxystearamide wax, or hydroxy bis-stearamide waxes. Other synthetic ornatural waxes, for example, carnauba wax, esparto wax, etc. can also beconsidered. Examples of synthetic waxes, in addition to thoseaforementioned, are polyolefin waxes and other amide waxes. A suitablewax should have a high melting point of more than 100° C., preferably inthe range 110-150° C.

[0044] The wax significantly increases the flexibility of the product. Aseal of the same size as that above (1.9×4.8 cm, density 1.59) can bebent to a relatively small curvature (R=30 cm) and the product can beformed into a reel, for example, for shipping. As a general rule, enoughwax is used for the flexibility of the seal strip to be sufficient forshipping (bending onto a reel, in which the radius R<1.5 m). Even largerseal strips manufactured using the recipe according to the inventionmeet this criterion.

[0045] The use of synthetic graphite makes the seal anisotropic. Thegraphite is preferably used in the form of flakes or crystals.

[0046] The vulcanizing agent used in the example was Vulcup 40KE,Hercules, T-butylperoxy-diisopropyl benzene, but other vulcanizingperoxides, along with sulphur, can also be considered. Reactoraccelerators, for example, thiazols such as benzothiazyl sulphide (e.g.,MBTS, Vulkacit DM, Akrochem), or n-cyclohexylbenzo thiazyl sulphamide(Santocure MOR, Monsanto) can be used.

[0047] The seal strip (FlexSeal) manufactured using the method accordingto the invention was compared in several tests with known commerciallyavailable seal strips.

[0048] According to FIG. 2, the seal according to the invention wearsrelatively less than a conventional seal strip. As loading increases,the seal strip according to the invention behaved reliably compared to aconventional seal strip, which was observed to show a sudden increase inwear at a specific loading, if the loading increased further. The sealaccording to the invention has low wear in both wet and dry conditions.

[0049] According to FIG. 3a, the seal strip according to the inventiondemands less operating power than a conventional seal strip. Similarly,the level of noise production was considerably lower using a seal stripaccording to the invention compared to a conventional seal strip, FIG.3b.

[0050] Although the invention has been described by reference tospecific embodiments, it should be understood that numerous changes maybe made within the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but that it have the full scope defined by thelanguage of the following claims.

What is claimed is:
 1. A suction roll seal strip, which is manufacturedfrom a mixture, which mostly consists of nitrile rubber and graphite,characterized in that the mixture includes wax.
 2. A seal stripaccording to claim 1, characterized in that the amount of wax in themixture is set in such a way that the flexibility of the seal strippermits it to be bent onto a reel, the radius of which is less than 130cm.
 3. A method for manufacturing a suction roll seal strip from amixture, which mostly consists of nitrile rubber and graphite, and fromwhich mixture a seal blank is formed, which is hardened at a chosentemperature, characterized in that the mixture to be hardened includeswax.
 4. A method according to claim 3, characterized in that the amountof wax is 1-15%.
 5. A method according to claim 4, characterized in thatthe amount of wax is 2-4%.
 6. A method according to claim 3,characterized in that the amounts of nitrile rubber and graphite arewithin the ranges: nitrile rubber 30-60% volume, and graphite 30-60%volume.
 7. A method according to claim 3, characterized in that thehardening temperature is in the range 140-160° C., and the hardeningtime is in the range 3-5 hours.
 8. A method according to claim 7,characterized in that the hardening temperature is in the range145-155°.
 9. A method according to claim 3, characterized in that themass includes 1-15% sulphur or peroxide.
 10. A method according to claim3, characterized in that the graphite used is natural graphite.
 11. Amethod according to claim 3, characterized in that the graphite used issynthetic graphite.
 12. A method according to claim 3, characterized inthat the melting point of the wax used is more than 100° C.
 13. A methodaccording to claim 12, characterized in that the melting point of thewax used is within the range 110-150° C.
 14. A method according to claim3, characterized in that the amount of wax is arranged to createflexibility in the seal strip, which will permit it to be rolled onto areel with a radius of less than 1.5 m.